The present invention relates to an electrophotosensitive material which is used in image forming apparatuses such as electrostatic copying machine, laser beam printer and facsimile and, more particularly, to an electrophotosensitive material which is excellent in durability, particularly wear resistance and gas resistance.
In image forming apparatuses such as electrostatic copying machine, facsimile and laser beam printer, electrophotosensitive materials having sensitivity in a wavelength range of a light source used in these image forming apparatuses are used. As the electrophotosensitive material, an inorganic photosensitive material having a photosensitive layer made of an inorganic material such as selenium and an organic photosensitive material (or organic photoconductor, OPC) having a photosensitive layer made of an organic conductive material are known. Among these photosensitive materials, the latter organic photosensitive material has widely been studied, recently, because it is easily produced as compared with the inorganic photosensitive material and has a wide range of choice of photosensitive materials such as electric charge transferring material, electric charge generating material and binder resin as well as high functional design freedom.
To maintain quality of the formed images in a good state for a long period in the above image forming apparatuses, it is required to enhance durability of the electrophotosensitive material to an external force applied during processes of charging, toner development, transfer onto papers and cleaning, and particularly to further improve wear resistance of a photosensitive layer.
In the organic photosensitive material, therefore, it has been studied to use a polycarbonate resin having high mechanical strength as a binder resin for formation of the photosensitive layer (see Patent Documents 1 and 2).
As typical examples of a polyallylate, which is a polyester of diphenol, “U-polymer (R)” as a condensate of bisphenol A and phthalic acid is known. It is described that this polyallylate is suited for use as a binder resin for improving the durability of the photosensitive layer because of its high heat distortion temperature and excellent mechanical properties (see Patent Document 3).
Furthermore, as a binder resin capable of forming a photosensitive layer having excellent durability as compared with the “U-polymer (R)” and polycarbonate, for example, polyallylate (Patent Documents 4 and 5) wherein a diphenol component is fluorenone type bisphenol or 1,1-bis(4-hydroxyphenyl)-1-phenylethane, or polyallylate a diphenol component is bisphenol C or 1,1-bis(4-hydroxyphenyl) cycloalkane and the resin has a viscosity within a predetermined range (Patent Documents 6 and 7) is disclosed.
Patent Document 1: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Sho) No. 63-261267 (see claim 1)
Patent Document 2: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Sho) No. 59-71057 (see claim 1)
Patent Document 3: Japanese Published Unexamined Patent Application (Kokoku Tokkyo Koho Sho) No. 61-55672 (see claim 1)
Patent Document 4: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Hei) No. 5-297601 (see claim 1, Example 1 and Comparative Examples 1 to 2)
Patent Document 5: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Hei) No. 5-341539 (see claim 1, Example 1 and Comparative Examples 1 to 2)
Patent Document 6: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Hei) No. 9-22126 (see claim 1 and Comparative Example 3)
Patent Document 7: Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho Hei) No. 8-110646 (see claim 1, paragraph [0010], paragraph [0011] and Comparative Examples 3 to 4)
However, the effect of improving the mechanical strength of the photosensitive layer is insufficient at present. Even when using the binder resins described in the above publications, the photosensitive layer causes wear within a comparatively short period when used practically in an image forming apparatus, and thus it is insufficient to achieve longer life of the electrophotosensitive material.
During charging of the electrophotosensitive material, an ozone gas or a NOx gas is sometimes evolved by corona discharge. Because of insufficient ozone gas or NOx gas shielding properties, a photosensitive material made of a conventional binder resin causes a problem wherein chargeability of the photosensitive material is deteriorated by exposure to the gas for a long time, namely, deterioration of charge stability during repeated exposure.